1. Field of Invention
This invention relates to optical communication systems, and more particularly to improvements thereof.
2. Description of the Prior Art
FIG. 1 is a block diagram depicting a conventional optical communication system of the multi-drop type. Although only one-way transmission is shown, in practice, two way transmission is used. Stations ST11, ST12, . . . , ST1N are respectively connected by optical coupling devices A11, A12, . . . , A1N, to an optical transmission line L1.
FIG. 2 is an explanatory view depicting an optical coupler used as the device A11, . . . , A1N of FIG. 1. Optical input signals I11 and I12 are respectively separated into two paths by a half mirror 10, to produce optical output signals O11 and O12. Assuming the optical transmittance of the half mirror 10 to be .alpha.1, there then exists the following relationship: ##EQU1## and the coupling ratio is fixed.
Since this system is comprised of passive elements, disadvantageously, a large attenuation occurs between the transmitting stations and the receiving stations. On the other hand, this system provides some degree of reliability, acceptable transmission delay and error rate, as compared with a loop type optical communication system for performing regenerative repeating.
The maximum attenuation results when the transmission and reception between stations ST11 and ST1N, has a transmission gain G1 which can be represented by the following: EQU G1=.alpha.1.sup.2 (1-.alpha.1).sup.N-Z
The gain is maximum at .alpha.1=N/2, which is substituted into the above equation to express the maximum transmission gain Gt1 by the following equation: ##EQU2##
That is, since the attenuation increases in proportion to N.sup.2, it can not actually be practiced if the scale of the system is enlarged.
In the conventional loop type optical communication system using an optical switch as the optical coupling device, the regenerative repeating is performed when each of the stations operates normally, that is, when a fundamental 1:1 transmission is carried out. If there should occur an abnormality in the operation of a station (for example, disconnection of a power source), the optical switch is suitably operated to bypass the relevant station. In this prior system, although attenuation between transmitting and receiving stations is not particularly troublesome, there is a deficiency in that a delay is enlarged due to the repeating regeneration of signals and communication errors are accumulated to increase the error rate.
Furthermore, in the conventional multi-drop type optical communication system using optical couplers, repeaters are usually inserted in the optical transmission line in order to recover the attenuation of the signals accompanying the increase in the number of nodes (each of the nodes comprises an optical coupling device, a station and the like) and the distance covered. Accordingly, although the optical coupling device for each of the stations is composed of passive elements and hence can provide good reliability, reduced transmission delay and low error rate, these characteristics are impaired at the portion of the repeater whereat active elements are used.